Linear Image Sensor Readers: The Rise of CCD and CIS Technology in Barcode Scanning |
Executive Summary |
This article provides a comprehensive exploration of linear image sensor readers, focusing on CCD (Charge-Coupled Device) and CIS (Contact Image Sensor) technology in barcode scanning applications. We examine how these sensor arrays capture an entire line of the barcode at once, eliminating the need for moving mirrors and enabling more compact, reliable, and cost-effective reader designs. Rather than focusing on abstract theory, we ground every concept in concrete design examples and real products from industry leaders including Toshiba, Sony, Cirrus Logic, and Newland. We explore the fundamental architecture of CCD and CIS sensors, the role of analog front ends (AFEs) and correlated double sampling (CDS) in noise reduction, the practical implementation of timing generation, and the application of these sensors in both handheld and fixed-mount readers. The article covers both the fundamental principles and the practical implementation details that make linear image sensors the preferred choice for many modern barcode reading applications. The closing summary synthesizes the key lessons and offers practical guidance for anyone designing or selecting linear image sensor systems for barcode reading. |
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Chapter 1: The Principle of Linear Image Sensing |
Unlike laser scanners that use a moving mirror to sweep a single spot of light across a barcode, linear image sensors capture an entire line of the barcode in a single instant. They are essentially one-dimensional cameras: an array of photodiodes, each corresponding to a pixel, that converts the reflected light from the barcode into an electrical charge. This charge is then read out sequentially, creating a time-varying signal that represents the barcode pattern. |
The linear image sensor offers several fundamental advantages over laser scanning. First, it has no moving parts, making it more reliable and durable, especially in portable devices. Second, it captures the entire barcode at once, so it is not dependent on the user's scanning speed. Third, it can be made very compact, fitting into the slim form factors of modern handheld readers. |
A Chinese-language research paper on CIS-based barcode readers explains the fundamental operation: 'The light source array emits light that is directly projected onto the surface of the object being scanned. The light reflected from the object's surface is focused by a self-focusing rod lens array onto the photoelectric sensor array, where it is converted into charge and stored. Different parts of the object have different light intensities, so different sensor units receive different light intensities. Each pixel of the CIS corresponds to a point on the object's image, and the entire sensor array records the image information of the object' . |
There are two main types of linear image sensors used in barcode readers: CCD (Charge-Coupled Device) and CIS (Contact Image Sensor). Each has its own characteristics and is suited to different applications. |
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Chapter 2: CCD Linear Image Sensors |
The CCD (Charge-Coupled Device) is the older and more established technology. A CCD sensor consists of an array of photodiodes that convert light into charge, and a shift register that transfers this charge out of the device for readout. The charge is transferred from one pixel to the next in a 'bucket brigade' fashion, hence the name 'charge-coupled.' |
CCD sensors are known for their high sensitivity, low noise, and excellent image quality. They are commonly used in high-end document scanners, industrial inspection systems, and applications where image quality is paramount . However, they require multiple power supplies and complex timing signals, making the system design more challenging. |
Toshiba is a leading manufacturer of CCD linear image sensors for barcode readers. The TCD1103GFG is a CCD linear image sensor with 1500 image-sensing elements, a pixel size of 64 micrometers by 5.5 micrometers, and a data rate of 2 MHz. It is specifically designed for barcode reader applications . The sensor has a sensitivity of 79 V/lx.s, a dynamic range of 200, and operates from a single 3.3 V supply, consuming only 16 mW of power . |
Another popular CCD sensor from Toshiba is the TCD1304DG, which has 3648 pixels with a pixel size of 8 by 200 micrometers. It features an electronic shutter function (ICG) that keeps the output voltage constant with varying light intensities. This sensor has a sensitivity of 160 V/lx.s and a dynamic range of 300 . |
Sony also manufactures CCD sensors for barcode reading applications. The ILX554B is a 2048-pixel CCD linear sensor designed specifically for bar code POS hand scanners and optical measuring equipment. Key features include a built-in timing generator and clock drivers, enabling operation from a single 5 V power supply . The pixel size is 14 by 56 micrometers, and the sensitivity is 240 V/lx.s . The built-in timing generator simplifies the external circuitry required to drive the sensor. |
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Chapter 3: Contact Image Sensors (CIS) |
The CIS (Contact Image Sensor) is a more recent technology that integrates the light source, the photodetector array, and the focusing optics into a single, compact module. Unlike CCDs, which require external lenses and complex optical paths, CIS modules are designed to be placed in direct contact with the document being scanned. This integration makes them ideal for compact, low-power, and cost-effective applications. |
A CIS module typically consists of: an LED light source array for illumination, a rod lens array (often a self-focusing lens array) for focusing the reflected light, a linear photodiode array (either CCD or CMOS-based), and the supporting electronics for readout. The entire assembly is housed in a single module, simplifying the system design. |
The Chinese patent application US 2005/0139675 A1 describes a CIS-based card reader in detail. The CIS component module includes a light source, a connector for outputting analog signals, a photoelectric sensor array for accumulating optical signals, a printed circuit board for controlling the components, and a lens array for focusing the reflected light onto the sensor . When the SP signal is received, the light source illuminates the barcode, and the reflected light is focused onto the sensor array. The charges are then stored in a shift register and clocked out sequentially . |
CIS technology is particularly well-suited for handheld scanners because of its small size and low power consumption. The Newland EM1300 is a high-performance CCD-based scanning engine that uses a CMOS image sensor, but the principles are similar . Its small size and low weight (6.4 g) make it ideal for embedding into portable devices. |
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Chapter 4: The Analog Front End (AFE) |
The raw output from a CCD or CIS sensor is an analog signal that must be conditioned before it can be digitized. The analog front end (AFE) is the circuit that performs this conditioning. Typical functions include gain, offset adjustment, and correlated double sampling (CDS). The AFE converts the sensor's analog output into a clean, digitized signal that can be processed by the microcontroller. |
The Cirrus Logic CS82L46 is a high-performance, six-channel AFE specifically designed for scanner applications. It combines image sensor ADC, analog conditioning, and an LED driver in one device. It operates at pixel conversion rates up to 24 MSPS, making it suitable for high-speed document scanners and barcode scanning systems . |
The CS82L46 receives analog pixel data from the CCD or CIS sensor and performs: reset level clamping (RLC), correlated double sampling (CDS), gain and offset adjustment, and then converts the signal to digital pixel data for output to a scanner controller ASIC over CMOS or LVDS pins . It integrates RGB and white LED drivers and an on-chip LDO regulator for CIS support, reducing BOM and simplifying scanner front-end design . |
In the card reader described in US 2005/0139675 A1, the analog signals from the CIS are transmitted to an A/D converter (e.g., WM8150) where they are converted into 16-bit digital signals . These digital signals are then serially outputted to a CPLD, where they are latched and converted into 8-bit parallel signals for processing . |
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Chapter 5: Correlated Double Sampling (CDS) |
Correlated double sampling (CDS) is a noise reduction technique that is essential for achieving high-quality images from CCD and CIS sensors. It works by taking two samples of the pixel output: one during the reset phase (which contains the reset noise) and one during the signal readout phase (which contains the signal plus the noise). By subtracting the reset sample from the signal sample, the noise is canceled. |
The CS82L46 AFE includes CDS as a standard feature, performing reset level clamping (RLC) and correlated double sampling on each channel . This is crucial for CCD applications where reset noise is significant. In CIS applications, the CDS function can be bypassed or adjusted, as the noise characteristics of CIS sensors are different. |
A patent on signal processing for CCD imagers describes how CDS can be implemented with a comparator and hysteresis band. The comparator's hysteresis band is set just larger than the noise floor, and it trips when the filter output signal rises above the positive hysteresis threshold . This ensures that the comparator output is stable and noise-free. |
A research paper on a 2D image acquisition system notes that CDS is implemented in the AFE to reduce reset noise and fixed-pattern noise from the image sensor . This is a standard technique for achieving high-quality images from CCD and CIS sensors. |
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Chapter 6: Timing Generation and Control |
CCD and CIS sensors require precise timing signals to operate correctly. These signals include the start pulse (SP) that initiates a scan, the clock signal (CP) that shifts the data out, and various other control signals. The timing generator is responsible for producing these signals. |
In the CIS-based card reader described in US 2005/0139675 A1, a CPLD (Complex Programmable Logic Device) is used to generate the sequence signals required by the AFE and the CIS . The processor emits a master clock frequency (MMCLK), which is logically converted to a CIS-CLK signal by the CPLD. The SP signal is emitted from the processor and received by the CIS component module to initiate a scanning period . |
The research paper on CIS-based barcode readers describes the timing sequence: 'The CIS sensor can only work properly under the strict control of CP and SP signals... designing the driving timing of the CIS sensor is key to generating stable and reliable CP and SP signals, with perfect timing matching between them' . The paper notes that a CPLD-based driver has advantages in integration, speed, and reliability over other methods . |
The Sony ILX554B CCD sensor has a built-in timing generator and clock drivers, simplifying the timing design. The sensor operates from a single 5 V power supply, eliminating the need for multiple supply voltages . |
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Chapter 7: Integration and Signal Processing |
The integration of the analog and digital functions is a key trend in barcode reader design. The CS82L46 AFE is an example of this trend, combining six channels of analog conditioning with an ADC and LED drivers. The integration reduces the BOM and simplifies the board layout . |
In the card reader described in US 2005/0139675 A1, the CPLD not only generates timing signals but also processes the digital signals from the AFE. The 16-bit serial data from the A/D converter is latched, converted to 8-bit parallel data, and transmitted via DMA to the processor for decoding . This integration of timing generation and data processing reduces the burden on the processor. |
The processing in an imaging-based reader can be divided into two stages: image acquisition and decoding. The image acquisition stage captures the raw pixel data and conditions it. The decoding stage processes the conditioned data to extract the barcode information. |
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Chapter 8: Applications in Handheld and Fixed Readers |
Linear image sensors are used in a wide range of barcode reading applications, from handheld scanners to fixed-mount readers in checkout lanes and industrial settings. |
The Shenzhen Dyscan Technology DS5220-1D is a commercial handheld barcode scanner that uses a linear CCD sensor . The scanner features a 32-bit CPU, a resolution of 2500, and a decoding speed of up to 300 times per second . It has a depth of field from 20 mm to 680 mm and a print contrast signal of at least 20% . The linear CCD provides a cost-effective and reliable solution for retail, logistics, and inventory management. |
The Newland EM1300 is a barcode reading engine designed for OEM applications . It uses a CCD image sensor with a resolution of 2500 and a decoding speed of up to 300 times per second. It is designed to be embedded into various devices such as kiosks, ticket machines, and PDAs . Its small size (6.4 g) and low power consumption (40.7 mA typical, 99 mA max) make it ideal for battery-powered applications . |
The Toshiba TCD1103GFG and TCD1304DG are explicitly targeted at barcode reader applications, including POS hand scanners and optical measuring equipment . The Sony ILX554B is also designed specifically for bar code POS hand scanner and optical measuring equipment use . These sensors are suitable for both handheld and fixed-mount applications. |
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Chapter 9: Advantages of CIS and CCD Technology |
Both CIS and CCD technologies offer significant advantages over laser scanning. |
No moving parts: Linear image sensors are solid-state devices with no moving parts. This means they are more reliable and durable than laser scanners, which have oscillating mirrors or rotating polygons. There is no wear and tear, and they are less susceptible to damage from drops or impacts. |
Independent of scanning speed: A linear image sensor captures the entire barcode in a single line. The reading is not dependent on how fast the user moves the scanner. This makes them more user-friendly and reduces the chance of a failed read due to scanning too fast or too slow. |
Compact form factor: CIS modules, in particular, can be made very thin and light. This makes them ideal for compact handheld readers and embedded applications. |
Lower cost: Linear image sensors are generally lower-cost than laser scanners, especially for the component-level integration and the simple PCB design required. The elimination of complex moving parts reduces the manufacturing cost. |
High resolution: Linear image sensors can have a very high pixel count (e.g., 5340 pixels in the Toshiba TCD1705AD), enabling the reading of very small barcodes and high-density symbologies . |
Good reading of challenging surfaces: Linear image sensors can read barcodes on curved surfaces or reflective materials, where a laser scanner might struggle. |
Low power consumption: CIS modules, in particular, consume very low power, making them suitable for battery-powered handheld devices. The Newland EM1300, for example, has a typical operating current of only 40.7 mA . |
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Chapter 10: Future Trends |
The future of linear image sensors in barcode readers is likely to be driven by continued improvements in resolution, speed, and integration. The trend toward higher resolution will enable the reading of increasingly smaller barcodes. The trend toward faster data rates will enable faster scanning and decode times. |
The integration of the sensor, AFE, and processor on a single chip is a key trend. This will further reduce the size, cost, and power consumption of barcode readers. |
The use of CMOS image sensors, rather than traditional CCD sensors, is also likely to increase. CMOS sensors offer several advantages, including lower power consumption, lower cost, and easier integration with other circuits. Many modern CIS modules already use CMOS sensors. |
Finally, the line between linear image sensors and area image sensors (2D imagers) is blurring. Many modern readers can read both 1D and 2D barcodes, using a 2D area imager that captures a complete image of the barcode. The principles of image acquisition and signal processing are similar, whether the sensor is a linear or area array. |
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Chapter 11: Summary --- Linear Image Sensor Readers in Perspective |
Linear image sensor readers represent a significant shift in barcode scanning technology, offering a reliable, compact, and cost-effective alternative to laser scanners. By capturing an entire line of the barcode in a single exposure, they eliminate the need for moving mirrors and provide a more user-friendly experience. |
We have examined how different companies and technologies have approached the challenges of linear image sensor design: |
Toshiba offers a range of CCD linear image sensors for barcode readers, including the TCD1103GFG with 1500 pixels, the TCD1304DG with 3648 pixels, and the high-resolution TCD1705AD with 5340 pixels . These sensors are designed for high sensitivity, low noise, and high-speed scanning. |
Sony provides the ILX554B, a 2048-pixel CCD linear sensor with a built-in timing generator and clock drivers, simplifying the system design for bar code POS hand scanners . The sensor operates from a single 5V supply, reducing the power supply requirements. |
Cirrus Logic offers the CS82L46, a six-channel analog front end (AFE) for scanner applications. The AFE integrates correlated double sampling (CDS), gain and offset adjustment, and an LED driver in one device, reducing BOM and simplifying scanner front-end design . |
Newland provides the EM1300, a high-performance CCD scanning engine designed for OEM embedding. The engine features a small form factor (6.4 g), low power consumption (40.7 mA), and a decoding speed of up to 300 times per second . |
US Patent 2005/0139675 A1 describes a CIS-based insert-type barcode reader, with a detailed block diagram including a CPLD for timing generation and signal processing, and a processor for decoding the barcode data . |
US Patent 5,814,803 describes signal processing and rescanning techniques for CCD bar code readers, including the use of FIR filters and analog shift registers for preprocessing data to reduce the burden on the decoder . |
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The key lessons from our exploration are: |
Linear image sensors capture an entire line of the barcode at once. This eliminates the need for moving parts and makes the reader independent of scanning speed. |
CCD sensors offer high sensitivity and low noise. They are used in high-end applications where image quality is critical. They require multiple power supplies and complex timing signals. |
CIS sensors integrate the light source, optics, and sensor in a single module. They are compact, low-power, and cost-effective, making them ideal for handheld and embedded applications. |
The analog front end (AFE) is crucial for signal conditioning. It performs correlated double sampling (CDS) to reduce noise, gain and offset adjustment to optimize the signal level, and analog-to-digital conversion. |
Precise timing generation is essential. CCD and CIS sensors require specific timing signals (start pulse, clock signal) to operate correctly. CPLDs or dedicated timing generators are often used for this purpose. |
Integration is a key trend. The sensor, AFE, and processing functions are increasingly being integrated into single devices or modules, reducing size and cost. |
Linear image sensors are well-suited for a wide range of applications. They are used in both handheld readers for retail and logistics, and in fixed-mount readers for industrial and automation systems. |
In the end, linear image sensor readers are a testament to the power of solid-state technology. They have transformed the barcode reading industry, making scanners more reliable, compact, and affordable. The art of linear image sensor design lies in the careful selection and integration of the sensor, AFE, timing generation, and processing components, creating a reader that can capture and decode barcodes quickly and reliably in any environment. |
cs@easiersoft.com